ConceptMechanics
← All conceptsKinematics & motion graphs
displacement vs distance (scalar/vector), velocity vs speed, acceleration & sign conventions; uniformly accelerated motion (v=u+at, s=ut+1/2at2, v2-u2=2as); free fall; v-t and x-t graph interpretation (slope vs area)
18 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- Acceleration zero at apexAssumes acceleration becomes zero at the top of a vertical throw rather than remaining g downward.→
- Average velocity as endpoint meanComputes average velocity as (vinitial+vfinal)/2 for non-uniform acceleration where it does not apply.→
- Below-axis area sign ignoredAdds the area below the time axis on a v-t graph as positive distance when computing net displacement.→
- Constant velocity implies accelerationBelieves a moving object at constant velocity still has nonzero acceleration, or that zero velocity means zero acceleration.→
- Displacement vs distance confusedTreats total path length (distance) as the vector displacement, or returns displacement when distance is asked.→
- Heavier objects fall fasterAssumes a more massive object has greater free-fall acceleration, ignoring that g is mass-independent.→
- Inconsistent sign for gMixes up-positive and down-positive conventions within one problem, mis-signing g relative to displacement and velocity.→
- Instantaneous vs average velocityUses average velocity over an interval as if it were the instantaneous velocity at a point.→
- Negative acceleration means slowingAssumes negative acceleration always means decelerating, ignoring that it speeds up an object moving in the negative direction.→
- Path length used for sSubstitutes total distance travelled for s when the equation needs net displacement (e.g. a throw that returns down).→
- Return speed differs from launchBelieves the speed returning to launch height differs in magnitude from the launch speed, ignoring symmetry.→
- Up/down free-fall asymmetryTreats time to rise and time to fall (same height, no air resistance) as unequal, breaking the symmetry.→
- Velocity nonzero at top of throwThinks the velocity at the highest point of a vertical throw is nonzero rather than momentarily zero.→
- Velocity vs speed confusedReports average speed when average velocity (displacement/time) is required, ignoring direction reversals.→
- suvat used with changing accelerationApplies the constant-acceleration suvat equations to a situation where the acceleration is not constant.→
- v-t area/slope swappedUses the slope of a velocity-time graph for distance, or its area for acceleration, swapping the two operations.→
- v2=u2+2as mis-squaredForgets to square u or v, or drops the factor of 2, when using v2-u2=2as.→
- x-t value read as velocityReads the height of a position-time graph as the velocity instead of using its slope.→
